Add initial test files from lodash v4. (#4172)

* Install test dependencies.

* Add initial test files.

These files were created using a simplistic AST manipulator using `recast` to
preserve formatting. There's bound to be a huge chunk of errors, but this serves
as a good start. QUnit was replaced with Mocha, with ES2015 imports running via
`esm`.

As far as possible, QUnit-specific syntax has been replaced with Mocha's
`describe` and `it`, while the native Node.js `assert` module is used for
assertions. Files in the `test` directory ending in `.test.js` will be treated
as test files.

* Add initial passing files to test run.

test/Strings-category-methods.js

@@ -0,0 +1,43 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { _, stubString } from './utils.js';
+
+describe('"Strings" category methods', function() {
+  var stringMethods = [
+    'camelCase',
+    'capitalize',
+    'escape',
+    'kebabCase',
+    'lowerCase',
+    'lowerFirst',
+    'pad',
+    'padEnd',
+    'padStart',
+    'repeat',
+    'snakeCase',
+    'toLower',
+    'toUpper',
+    'trim',
+    'trimEnd',
+    'trimStart',
+    'truncate',
+    'unescape',
+    'upperCase',
+    'upperFirst'
+  ];
+
+  lodashStable.each(stringMethods, function(methodName) {
+    var func = _[methodName];
+
+    it('`_.' + methodName + '` should return an empty string for empty values', function() {
+      var values = [, null, undefined, ''],
+          expected = lodashStable.map(values, stubString);
+
+      var actual = lodashStable.map(values, function(value, index) {
+        return index ? func(value) : func();
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    });
+  });
+});